Our long term goal is to learn how the self-renewing ability of stem cells is achieved - a question central to the understanding of tissue development, maintenance, and repair, with profound implications in regenerative medicine and cancer treatment. Our strategy is to use Drosophila as a pioneer model to address this question, and to extend what we learn from Drosophila to mammalian and clinical settings. We previously identified germline stem cells in Drosophila and revealed the self-renewing pattern of their division. We then discovered key genes that define inter- and intracellular mechanisms of stem cell division, including the piwi (a.k.a., argonaute) family genes~the only known family of genes with stem cell function highly conserved during evolution in both animal and plant kingdoms. Our demonstration of the essential role of intercellular signaling in germline stem cell maintenance has led to the development of the current stem cell niche theory; whereas our analyses of the piwi family genes start to link epigenetic regulation to stem cell function. Our future focus will be on how epigenetic mechanisms dictate stem cell fate. We will investigate what regulates changes of chromatin organization and activity to generate the unique gene expression profile of the genome that defines stem cell fate. Specifically, we are in a unique position to study epigenetic programming mediated by PIWI family proteins and micro RNAs ~ an exciting new frontier of stem cell research. Our working hypothesis is that PIWI controls stem cell fate by regulating the transcriptional activity of the chromatin, possibly via micro RNA-mediated process. This regulation can occur in stem cells or in niche cells. To test and further develop this hypothesis, we plan to accomplish the following aims: 1. Test the fundamental hypothesis that epigenetic programming determines stem cell fate. 2. Define and characterize the functional domains of PIWI. 3. Identify PlWI-interacting proteins and characterize their epigenetic role in the stem/niche cell genomes. [unreadable] [unreadable] [unreadable]